The subject matter of the disclosure relates to a printhead support structure and a print assembly permitting movement or positional adjustment of a printhead, and a method of adjusting a position of a printhead coupled to a printhead support.
Printers are well-known devices for applying text and graphic images to a variety of substrates. A wide variety of different printers are available which are suitable for printing onto different types and sizes of substrate.
Large-scale industrial printers are adapted to print images onto larger substrates than, for example, office-based printers used for printing onto A4-size paper. Large-scale printers may be used for printing onto, for example, advertising boards, posters, and/or large batches of smaller substrates.
In an inkjet printing process, a series of droplets of, for example, ink is deposited onto the surface of a substrate in a pattern to form the required image. The droplets of ink are typically emitted from nozzles on an inkjet printhead. A typical printer includes several printheads arranged along a print carriage. The print carriage can be up to around 2 m in width. Printer manufacturers aim to provide a dense and continuous array of printheads across the whole width of the print carriage. Usually these are provided in multiple rows to give a 2-D array of printheads.
Recent advances in inkjet printhead manufacture have allowed manufacturers to integrate several thousand inkjet nozzles on a single printhead: this is frequently achieved by arranging the nozzles in a two-dimensional grid pattern, as illustrated schematically in FIG. 1.
In order to achieve good positional registration (i.e. relative positioning) between nozzles within a printhead, the correct azimuthal rotation of the printhead should be established. This is illustrated in FIG. 1, which shows lines of ink 20a, 20b, 20c laid down by printhead nozzles 10a, 10b, 10c. When the printhead is correctly rotationally aligned, the lines of ink 20a, 20b, 20c laid down by the nozzles 10a, 10b, 10c are equally spaced. However, if the array of nozzles 10a′, 10b′, 10c′ is rotated and incorrectly rotationally aligned, the printed lines of ink 20a′, 20b′, 20c′ are no longer equally spaced.
In addition to the azimuthal rotation of the printhead, translational alignment in the print direction (“along-process” direction i.e. in the direction the print carriage moves in relation to the substrate) and perpendicular to the print direction (“cross-process” direction i.e. across the width of the print carriage) should also be considered. In order to maintain the equal spacing of the printed lines of ink at the boundaries between printheads, there should also be good registration between printheads in the cross-process direction.
Along-process registration may be achieved by altering the firing times of the individual printheads, as is illustrated in FIG. 2, and vertical positioning and other rotations can be set adequately by manufacturing tolerances. FIG. 2 shows the nozzles of a first printhead 1 and a second printhead 2, which are not aligned in the along-process direction. Registration between these can be achieved by delaying the firing time of printhead 2 compared to printhead 1, so that both arrays of nozzles lay down ink in the same place on the substrate.
However, as printheads with higher resolutions and smaller drop sizes are developed, azimuthal and cross-process positioning are difficult to achieve using standard manufacturing tolerances so some degree of mechanical adjustment can be used to enable alignment of the printheads within the print carriage.
Printheads are usually manufactured individually and fixed to a print carriage, on which they are aligned. Some printheads are modular, with every printhead individually replaceable in the field, requiring them to be individually adjustable for alignment. While technically challenging, this can provide improvements in the accuracy of alignment, because there is no stack up of tolerances, and because the final adjustment is done with the head in its operating condition. This also means that the final printed position of droplets is used for alignment, rather than nozzle position, so it includes any systematic jet deviations. A typical print carriage may have around 150 printheads, and the initial aligning and maintaining the alignment of that number of printheads is quite a demanding task.
Further, when building large arrays of printheads, it is desirable to make the assembly as compact as is reasonably possible, as this improves the registration between printheads both within and between colours. However, this also means it is more difficult to make adjustments.
Aspects of the invention are set out in the independent claims and preferred features are set out in the dependent claims.
There is described herein a printhead support structure, comprising: means for receiving a printhead; first and second portions having adjustment means therebetween for converting a translational movement of the first portion to a rotational movement of the second portion; and means for coupling the second portion to said receiving means for adjusting the rotational angle of the printhead.
By providing a printhead support structure that can use a translational actuation to provide a rotational adjustment of the printhead, a printhead can be rotationally aligned after installation, even in a tightly packed array where space restrictions can make it difficult to provide a rotational actuation to individual printheads. Advantageously, it is easier to achieve good alignment when the printhead is adjusted in its operating position, since it is possible to compensate for discrepancies in the manufacture of other components in a printer, such as the support structure, print carriage and/or print table.
Preferably, the first portion is coupled to a print carriage and constrained to move substantially along a first axis; and the second portion is fixed at an edge, such that the second portion is constrained to rotate about a second axis parallel to the first axis.
Preferably, the second portion is fixed at the edge by means of a flexure.
Using a flexure to fix the second portion is advantageous because flexures are very stable and resilient to thermal changes and vibration. They do not exhibit “slop” or “backlash” and do not require locking. Additionally, it is possible to cut flexures out of the existing printhead support structure, so no further parts or material is required.
Preferably, the adjustment means is arranged such that a translational movement of the first portion along a first axis produces a force on or causes a force to be applied to the second portion in a direction perpendicular to the first axis, such that said force causes the second portion to rotate about a second axis parallel to the first axis.
By arranging the adjustment means in this way, the translational movement of the first portion substantially along the first axis transfers a force to the second portion to cause movement of an edge of the second portion in a direction substantially perpendicular to the first portion. When an opposing edge of the second portion is fixed, this causes the second portion to rotate about this fixed edge.
Preferably, the second portion is coupled to the printhead such that the rotational movement of the second portion about a second axis provides a rotational movement of the printhead about an axis parallel to the second axis.
Preferably, the adjustment means comprises a flexure arrangement.
By using a flexure arrangement for the adjustment means, it is possible to reduce the frequency with which printheads need to be realigned because flexures are very resilient to thermal changes and vibration. It has been unexpectedly found that such a flexure arrangement is very stable and so frequent readjustment does not seem to be required. Additionally, the use of a flexure means locking is not required, since flexures do not have backlash or slop, unlike, for example, sliding hinges.
Preferably, the flexure arrangement comprises two or more flexures.
By using two or more flexures, the translational movement in the first direction can cause the adjustment means to bend at these two flexure points, and hence produce a force in a perpendicular direction.
Preferably, the flexure arrangement is formed within the body of the printhead support structure.
By forming the flexure arrangement from the body of the printhead support, in particular by removing parts of the structure to form flexures, the adjustment mechanism does not require any extra space in the print carriage or any additional material and so the solution can be implemented cost-effectively and it is possible to place the printheads in a tightly packed array and to keep the print carriage fairly compact.
Preferably, the flexure arrangement comprises a pair of opposed flexure points with a diagonal linkage.
By providing a diagonal linkage between two opposed flexure points, it is possible to use a geometrical reduction to convert a relatively large translational movement into a finer/smaller rotational movement.
Preferably, the printhead support structure retains the printhead in a fixed position after adjustment without an additional locking mechanism.
By providing a printhead adjustment structure which does not require locking to keep a printhead in place, a more precise adjustment can be made because locking normally produces some movement, which changes the alignment made during the adjustment stage. It is necessary to compensate for any change due to locking when making the adjustment, prior to locking. Therefore, several attempts (e.g. “trial and error”) may need to be made before the correct adjustment is found. Such multiple attempts in adjustment are not necessary when locking is not required. Adjustments that do not require locking are also easier to automate.
Preferably, the second portion is fixed at a first edge, such that a second edge of the second portion, opposed to the first edge, is constrained to rotate about the first edge. The rotational movement of the second edge has a component perpendicular to the plane of the second portion and the adjustment means is arranged to provide a reduction ratio such that the magnitude of this component of movement of the second edge of the second portion and the magnitude of the translational movement of the first portion are in a ratio of less than one. The component of movement of the second edge that is perpendicular to the plane of the second portion may be termed herein the translational movement of the second portion.
Preferably, the adjustment means is arranged to provide a reduction ratio such that the rotational movement of the second portion and the translational movement of the first portion are in a ratio of less than one.
By arranging the adjustment means such that the rotational movement, or the magnitude of the translational movement caused by the rotation of the opposing or outer edge of the second portion is smaller than that of the translational movement of the first portion, very small, accurate adjustments can be made to the alignment of the printhead. Additionally, any forces on the printhead will only produce relatively small forces at the adjustment mechanism, which enables the adjustment to be much more stable during use and removes the need for frequent readjustment or locking. Furthermore, by providing a reduction ratio in the adjustment, any small movement of the printhead adjustment elements (i.e. the first portion, screws, pivots), caused by vibration, changing loads or thermal cycling during printer operation would only be transferred to the printhead in a ratio of less than one.
Preferably, the printhead support structure further comprises an adjuster screw arranged such that rotation of the adjuster screw provides said translational movement of the first portion.
By providing a screw for actuating printhead adjustment, the accuracy of adjustments can be improved because a relatively large rotation of the screw produces a smaller translational movement of the screw. Additionally, the screw can stay fixed in place once an adjustment has been made without the requirement for locking, for example due to the friction created by the thread of a screw. Furthermore, it is easy to automate the actuation of a screw, for example by using a motor.
Preferably, the printhead adjustment is actuated from a direction parallel to the axis of rotation of the printhead.
When printheads are closely packed in a large array, it is much easier to access each printhead from above or below the plane of the printhead array than from a direction adjacent to the printhead. Therefore, it is advantageous to be able to actuate a rotation in the plane of the printhead from a direction parallel to the axis of rotation.
Preferably, the printhead has an array of a plurality of nozzles and the rotational movement of the printhead is in the plane of the array of nozzles.
By rotating a printhead in the plane of the nozzle array, the correct azimuthal rotation of the printhead can be found to ensure that lines of ink laid down by the nozzles are equally spaced.
Preferably, the mechanism is further operable to provide a translational movement of the printhead.
By providing a mechanism which can provide a translational movement to the printhead, it is possible to adjust the position of printheads relative to other printheads within a printhead array and/or relative to the print carriage. Such adjustments can be helpful to achieve correct relative positioning of the nozzles between printheads.
Preferably, the translational movement provided to the printhead is in the cross-process direction.
By providing a translational adjustment to the printhead in the cross-process direction, the spacing between lines of ink laid down by nozzles on adjacent printheads can be adjusted. This can help to ensure consistent density of ink across the width of the substrate (i.e. perpendicular to the print direction).
Preferably, the printhead support structure further comprises a third portion coupled to the printhead such that a translational movement of the third portion provides said translational movement of the printhead.
Preferably, the translational movement of the printhead compensates for an alteration in the translational position of the printhead effected by said adjusting of the rotational angle of the printhead.
By providing means for compensating for the translation caused by rotational movement in the printhead support which provides the rotational movement, correct complete alignment of the printhead can be achieved in a single set of adjustments.
Preferably, the translational movement of the printhead alters the effective axis of rotation of the printhead.
The desired printhead rotation may be about an axis that is different from the axis the second portion causes the printhead to rotate about. Therefore, in order to achieve the desired printhead adjustment, it may be necessary to provide an additional translational movement.
Preferably, the printhead support structure further comprises a translational motor for effecting translational movement of the third portion.
Preferably, the printhead support structure further comprises a translational adjuster screw arranged such that rotation of the adjuster screw provides translational movement parallel to the direction of the axis of rotation of the printhead; and wherein the adjuster screw is in communication with the third portion, such that the translational movement provided by the screw is transferred to the third portion.
Preferably, the translational motor is in communication with the translational adjuster screw and wherein the translational motor is operable to rotate the translational adjuster screw.
Preferably, the printhead support structure further comprises a motor for effecting translational movement of the first portion.
By providing motors for actuating/driving the adjustment mechanism, it is possible to automate the adjustment of printhead alignment, optionally from a distance or over a network. This can be more efficient, accurate and less error prone than performing adjustment manually (i.e. by a human operator physically adjusting the alignment). In addition, when printheads are closely packed within an array, it may be difficult for human operators to access the adjustment mechanism, and easier for a motor to operate in confined spaces.
Preferably, the motor is in communication with the adjuster screw and the motor is operable to rotate the adjuster screw.
By providing motors for rotating an adjuster screw, the amount the screw is rotated can be carefully controlled, in particular, to a greater degree of precision than when screws are rotated manually. For example, a stepper motor can be used, which provides rotation in steps of uniform, predetermined amounts (e.g. 1.8°).
There is further described herein a print assembly comprising an array of a plurality of printheads arranged in a plane; and a printhead support structure as described above for each of said plurality of printheads for adjusting the position of each printhead; wherein each printhead adjustment is actuated from a direction perpendicular to the plane of the printhead array.
By allowing printheads to be adjusted from above or below, the adjustment can be performed after printheads have been installed in a closely packed array. It is advantageous to have a large number of printheads in a closely packed array, as this leads to better print resolution, an improved registration between printheads both within and between colours or arrays and faster printing, but when closely packed, individual printheads cannot be accessed from within the plane of the array. By allowing adjustment of printheads after installation, the printheads can be individually replaced and then adjusted, which saves costs, rather than having to replace an entire array of printheads, which would need to be aligned prior to installation. Furthermore, printhead alignment can be adjusted to correct for alignment errors that occur during use of the printer after installation. Additionally, it is possible to adjust printhead alignment to correct for discrepancies in printer elements within standard manufacturing tolerances.
Preferably, the rotational movement of the printhead is in the plane of the printhead array.
There is also described herein a method for adjusting the position of a printhead coupled to a printhead support, comprising the steps of: applying a force to a first portion of the printhead support to effect a translational movement of the first portion; converting said translational movement of the first portion into a rotational movement of a second portion of the printhead support; and applying said rotational movement of the second portion to the printhead.
Advantages of this aspect and the optional features set out below correspond to those for the aspects already described above.
Preferably, the translational movement is provided substantially along a first axis; and the rotational movement is substantially about an axis parallel to the first axis.
Preferably, the method for adjusting the position of a printhead further comprises the step of receiving the printhead on the printhead support.
Preferably, the converting of translational movement to rotational movement is accomplished by means of a flexure arrangement.
Preferably, the method for adjusting the position of a printhead further comprises the step of: retaining the printhead in a fixed position after applying said rotational movement to the printhead without locking.
Preferably, the magnitude of the movement of an outside edge of the second portion and the magnitude of said translational movement of the first portion are in a ratio of less than one.
Preferably, the printhead comprises an array of a plurality of nozzles and the rotational movement of the printhead is in the plane of the array of nozzles.
Preferably, the method for adjusting the position of a printhead further comprises the step of: providing a translational movement of the printhead in a cross-process direction.
Preferably, the method for adjusting the position of a printhead further comprises the step of: calculating said translational movement of the printhead in the cross-process direction is calculated to compensate for the rotational movement applied to the printhead.
Preferably, the compensation for the rotational movement alters the effective axis of rotation of the printhead.
There is also described herein a method of manufacturing a printhead adjustment mechanism, comprising the steps of: providing a printhead support structure, the printhead support structure comprising means for receiving a printhead; and removing selected parts of the printhead support structure to form first and second portions and an adjustment means therein for converting a translational movement of a first portion of the printhead support structure to a rotational movement of a second portion of the printhead support structure; wherein the adjustment means is coupled to the receiving means so that rotational movement of the second portion effects the rotational angle of the printhead.
By removing selected parts of the printhead support structure to manufacture the printhead adjustment mechanism, the adjustment mechanism can be made very compact. This allows printheads to be closely packed together within and between arrays, which is advantageous because this leads to better print resolution, an improved resolution between printheads both within and between colours or arrays and faster printing.
Preferably, removing selected parts of the printhead support structure comprises removing a first segment of the printhead support structure to create a recess forming a first flexure point; and removing a second segment of the printhead support structure to create a recess forming a second flexure point; wherein said flexure points are arranged to convert translational movement of the first portion into rotational movement of the second portion.
Preferably, the two flexure points are arranged in a diagonal linkage.
Preferably, removal of the segments is performed by wire erosion or by cutting with a plunge cutter.
Preferably, the method of manufacturing a printhead adjustment mechanism, further comprises the step of removing a third section of the printhead support structure to create a third flexure point, wherein said third flexure point creates a flexure hinge arrangement for securing a printhead to the printhead support structure.
By creating a flexure hinge for clamping the printhead to the support structure, it is possible to attach the printhead securely to the support, without providing an additional locking mechanism, which would take up space in the printhead support structure and provide additional complexity to the system. Furthermore, it simplifies the manufacturing method, particularly if flexures are already being used in other parts of the printhead support structure, which means it is not necessary to provide separate equipment and/or processes for installing a different type of clamping mechanism in the printhead support.
There is also described herein a print assembly comprising: an array of a plurality of printheads arranged in a plane; and an adjustment mechanism for each printhead for providing a rotational adjustment about an axis perpendicular to the plane for adjusting the rotational alignment of each printhead; wherein the rotational adjustment is effected from a direction substantially parallel to the axis of the rotational adjustment.
When printheads are arranged in a closely packed array, it is difficult to access each printhead individually, and it is easiest and most efficient to access the printhead adjustment mechanisms from above or below the plane of the printhead array.
Preferably, a further translational adjustment is effected from the direction substantially parallel to the axis of the rotational adjustment.
There is also described herein a method for adjusting printhead alignment, comprising the steps of: determining the required printhead rotational adjustment; using said required printhead rotational adjustment to calculate the magnitude of a rotational correction required to perform said rotational printhead alignment; calculating the translational movement of the printhead which results from said correction required to perform said rotational printhead alignment; determining the required printhead translational adjustment in the cross-process direction; calculating the magnitude of a translational correction required to perform said translational printhead adjustment; wherein determining the required translational printhead adjustment comprises compensating for the calculated translational movement of the printhead which results from said correction required to perform said rotational printhead alignment; and applying said rotational and translational corrections to adjust the printhead.
By calculating the rotational adjustment required for a printhead and the translational movement which would result from it, and applying calculated rotational and translational corrections to the printhead, it is possible to achieve correct, or at least sufficiently accurate, printhead alignment in relatively few steps, since it negates the need to compensate through trial and error.
Preferably, said rotational and translational corrections are automated.
By automating the actuation of corrections, it is possible to perform quicker and more accurate printhead alignment than when adjustment is attempted manually.
Preferably, the method for adjusting printhead alignment further comprises calculating a compensation for along-process errors in printhead alignment.
By calculating a compensation for along-process errors, it is possible to ensure correct registration between printheads, and therefore that ink is laid down correctly on the substrate.
Preferably, compensating for along-process errors in printhead alignment comprises altering the firing times of neighbouring printheads
Preferably, calculating the required corrections comprises calculating the magnitude of the required movement of one or more printhead support portions.
Preferably, calculating the magnitude of the required movement of one or more printhead support portions further comprises calculating the required rotation of one or more adjustment screws.
Preferably, calculating the magnitude of the required movement of one or more printhead adjustment portions further comprises calculating the required steps to be performed by one or more motors.
Preferably, the method for adjusting printhead alignment is performed by a computer program.
Using these apparatus and methods, it has been found that the mechanical adjustments, and hence the registration of the printheads, can be made to resolutions of a few microns and are stable at that level, which achieves a good print quality.